In the past twenty years, charged droplets and strong electric fields have quietly revolutionized chemistry. In combination with an atmospheric-sampling mass spectrometer, charged droplets containing biomolecules or polymers have become a source for desolvated, gas-phase ions of these analytes. The process by which charged droplets evaporate is remarkably “soft” in that the process imparts little to no energy to the resulting ions. One technique, electrospray ionization (ESI), is the current “state of the art” for biomolecule mass analysis.
A schematic of the electrospray process is shown in FIG. 1. Here an analyte-containing liquid is pumped through a capillary needle. A high voltage source establishes an electric field between the needle and a plate. Sufficiently high voltages produce strong fields within the liquid itself drawing the liquid to a tip as it exits the needle. From this tip, commonly referred to as a Taylor cone, the liquid sprays outward as charged droplets that are accelerated in the electric field toward the plate. When the plate contains an inlet to a mass spectrometer, analyte within the charged droplets may be mass analyzed. (See, e.g., Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; John Wiley and Sons: New York, 1997, the disclosure of which is incorporated herein by reference.)
The phenomenon of electrospray was investigated long before it was applied to the study of gas phase ions transferred from solution. In 1750, French clergyman and physicist Jean-Antoine (Abbé) Nollet reported the earliest known reference to electrospray, over two hundred years before the term was coined. He demonstrated that water flowing from a vessel would aerosolize when the vessel was electrified and placed near electrical ground. He also observed that “a person, electrified by connection to a high-voltage generator, would not bleed normally if he were to cut himself; blood would spray from the wound.” (For discussion, see ORNL Review, State of the Laboratory, vol. 29 no. 1-2 1995, the disclosure of which is incorporated herein by reference) Roughly one hundred years later, Lord Kelvin designed an apparatus consisting of two liquid nozzles connected to opposite collection reservoirs. Small statistical differences in charging between water dripping from the nozzles quickly led to kilovolt differences and electrosprays at the nozzles. (See, e.g., Smith, J. N. Fundamental Studies of Droplet Evaporation and Discharge Dynamics in Electrospray Ionization, California Institute of Technology, 2000.)
In the early twentieth century, refined experimental techniques allowed for a more rigorous understanding of the electrostatics and electrodynamics of these phenomena. John Zeleny classified the formation of ethanol electrosprays through photographs. (See, e.g., Zeleny, J. Phys. Rev. 1917, 10, 1.) The sprays characterized in Zeleny's work are structurally similar to those employed for mass spectrometry today in which liquid is drawn into a conical shape before breaking into a fine mist of charged droplets. This work was followed by rigorous studies of the field-dependent deformation of soap films over cylindrical tubes by Wilson and Taylor. The conical shape of these films resembled Zeleny's observations of ethanol, and indeed has come to be termed the “Taylor cone” based on later theoretical work by G. I. Taylor. (See, e.g., Taylor, G. Proc. R. Soc. London, Ser. A 1964, 280, 383; and Taylor, G. Proc. R. Soc. London, Ser. A 1966, 291, 159, the disclosures of which are incorporated herein by reference.)
By the middle of the twentieth century, electrospray had become a popular painting technique. Reports by Hines, Tilney and Peabody, and several patents demonstrate the ease with which paint is atomized and applied to vehicles, housewares, and various metal goods. (See, e.g., Hines, R. L. J. Appl. Phys. 1966, 37, 2730; and Tilney, R.; Peabody, H. W. Brit. J. Appl. Phys. 1953, 4, S51, the disclosures of which are incorporated herein by reference.) However, it was not until 1968 that electrospray was introduced as a scientific tool. Dole and coworkers employed electrospray ionization to transfer high molecular weight polystyrene ions into the gas phase from a benzene/acetone solution. (Dole, M.; Mack, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. D.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240, the disclosure of which is incorporated herein by reference.) Their combination of electrospray and nozzle skimmer/pumping systems are similar to those employed today to transfer the charged species from atmospheric pressure into the vacuum system for analysis. Despite this novel approach, two decades would pass before the technique gained wide acceptance.
In the 1980s John Fenn and coworkers revolutionized the field of mass spectrometry by making electrospray easy and practical for the analysis of large biomolecules and polymers. Fenn's contributions to the body of knowledge surrounding electrospray culminated in his sharing of the 2002 Nobel prize in chemistry. Fenn and coworkers presented a series of papers that permanently established electrospray as a tool to introduce dissolved analytes including low molecular weight cationic clusters, negative ions, polyethylene glycol, and several biomolecules into the gas phase for mass analysis. (See, e.g., Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451; Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4671; Wong, S. F.; Meng, C. K.; Fenn, J. B. J. Phys. Chem. 1988, 92, 546; and Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64, the disclosure of which are incorporated herein by reference.) They noted that electrospray imparts multiple charges to large biomolecules and polymers thus lowering the m/z value allowing biomolecule analysis on mass spectrometers having only a modest m/z range. As a result of all this investigation and development, ESI-MS has become a popular tool for studying noncovalent interactions and characterizing biomolecules. Cole reviews the present state and the diverse applications of electrospray ionization. (Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; John Wiley and Sons: New York, 1997, the disclosure of which is incorporated herein by reference.) More recent research has considered a broad range of applications from the technologically important processes of electrostatic spraying to the revolution in mass spectrometric studies of biological molecules made possible by electrospray ionization. (See, e.g., Bailey, A. G. Electrostatic spraying of liquids; Wiley: New York, 1988; Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64; Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984, 88, 4451; Electrospray Ionization Mass Spectrometry; Cole, R. B., Ed.; John Wiley and Sons: New York, 1997; Kebarle, P. J. Mass Spectrom. 2000, 35, 804; Grimm, R. L.; Beauchamp, J. L. Anal. Chem. 2002, 74, 6291; and Smith, J. N.; Flagan, R. C.; Beauchamp, J. L. J. Phys. Chem. A 2002, 106, 9957, the disclosures of which are incorporated herein by reference.)
Although the electrospray process is relatively straightforward to implement as an analytical tool for biomolecule mass analysis, research continues to elucidate the mechanisms by which an analyte-containing electrospray generates charged droplets that ultimately result in desolvated, charged, gas-phase molecules. In 1882, Lord Rayleigh considered the electrical pressure resulting from excess charge q on a droplet of spherical radius r and surface tension σ. His theory predicts that the natural quadrupolar oscillation of a droplet in an electrical field-free environment becomes unstable when q exceeds the limit qR, now known as the “Rayleigh limit”, defined in Equation 1.qR=8π(σε0r3)1/2  (1)The limit is reached either by solvent evaporation which decreases r or by application of charge in excess of qR. At q≧qR, Rayleigh postulated that the droplet would throw out liquid in “fine jets.” (See, Rayleigh, L. Philos. Mag. 1882, 14, 184, the disclosure of which is incorporated herein by reference.) This event is referred to in the literature as Rayleigh discharge or Coulomb fission. (See, e.g., Smith, J. N.; Flagan, R. C.; Beauchamp, J. L. J. Phys. Chem. A 2002, 106, 9957; Grimm, R. L.; Beauchamp, J. L. Anal. Chem. 2002, 74, 6291; and Kebarle, P. J. Mass Spectrom. 2000, 35, 804, the disclosures of which are incorporated herein by reference.) Despite a rigorous prediction of when the event occurs, Rayleigh's analysis does little to describe the dynamics of the discharge event.
Recent articles by Cole and by Kebarle and Peschke summarize the research performed to further elucidate the dynamics of a Rayleigh discharge. (See, Cole, R. B. J. Mass Spectrom. 2000, 35, 763; and Kebarle, P.; Peschke, M. Analytica Chimica Acta 2000, 406, 11, the disclosures of which are incorporated herein by reference.) FIG. 2 presents a schematic summary of the current view of the “lifetime” of a charged droplet. In the consensus view, charged, micrometer-sized droplets eject numerous progeny droplets having a diameter roughly one-tenth that of the parent. Other experiments and models such as the ion desorption model and the charge residue model address phenomena involving smaller droplets in the nanometer regime. Ultimately the result of ion desorption or the charge residue mechanism is a desolvated ion, or in some cases an ion-bound solvent cluster such as water. (See, e.g., Lee, S. W.; Freivogel, P.; Schindler, T.; Beauchamp, J. L. J. Am. Chem. Soc. 1998, 120, 11758, the disclosure of which is incorporated herein by reference.)
Table 1, below, summarizes the conclusions of Rayleigh discharge experiments found in the literature. This list is not inclusive, but rather shows the breadth and scope of the studies and their conclusions. Charged droplets generally undergo Rayleigh discharge while they are at 70-120% of their Rayleigh limit of charge (i.e. q=0.7 qR to 1.2 qR). For instance, Taflin and co-workers found discharge occurring below 90% the Rayleigh limit with charge loss ranging from 10 to 18%, and 1-2% mass loss in dodecaonol, hexadecane, heptadecane, dibromooctane, and bromodecane. (See, e.g., Taflin, D. C.; Ward, T. L.; Davis, E. J. Langmuir 1989, 5, 376, the disclosure of which is incorporated herein by reference.)
TABLE 1DROPLET CHARACTERISTICSDroplet% q/qR atDiam.Rayleigh% Mass% ChargeAuthor(s)SolventLocation(μm)DischargeLostLostLi, Tu, and Ray,diethyl phthalatebalance 5-25962.3212005diethylene glycol100<0.338triethylene glycol100<0.341hexadecane971.515Duft et al., 2003ethylene glycolbalance481000.333Duft et al., 2002ethylene glycolbalance 3-25>95n/a~25 Smith, Flagan, andwaterIMS10-60100n/a20-40Beauchamp, 2002methanol12015-20acetonitrile10015-20Feng et al., 2001methanolbalance20-42~100n/a80Widmann et al.,50 BTD, 50 IDDbalance25-3032421199750 BTD, 50 IDD17-2837574hexanediol diacrylate2264n/an/aGomez, Tang, 1994heptaneESI plume32-8060-80n/an/aRichardson et al.,n-octanolbalance 1-101022.3151989sulfuric acid 1-1084<0.149Taflin, Ward, andbromododecanebalance4472n/a12Davis, 1989dibromooctane26-40861.816dibutyl phthalate2075n/an/adocecanol36-3885215hexadecane28-64731.617heptadecane28-36791.612Roulleau et al., 1972waterbalance 50-200~100n/an/aSchweizer et al., 1971n-octanolbalance15-40~100523Berg et al., 1970balancebalance 30-250 25-100n/an/aAtaman et al., 1969n-octanolbalance30-60~100n/an/aDoyle, et al., 1964n/abalance 60-200n/an/a30Abbreviations:BTD: 1-bromotetradecane,IDD: 1-iodododecane,IMS: ion mobility cell,balance: electrodynamic balance,ESI: electrospray ionization,n/a: not available.References: Taflin, D. C.; Ward, T. L.; Davis, E. J. Langmuir 1989, 5, 376; Gomez, A.; Tang, K. Phys. Fluids 1994, 6, 404; Duft, D.; Lebius, H.; Huber, B. A.; Guet, C.; Leisner, T. Phys. Rev. Lett. 2002, 89, art. no. 084503; Li, K.-Y.; Tu, H.; Ray, A. K. Langmuir 2005, 21, 3786; Duft, D.; Atchtzehn, T.; Muller, R.; Huber, B. A.; Leisner, T. Nature 2003, 421, 6919; Feng, X.; Bogan, M. J.; Agnes, G. R. Anal. Chem. 2001, 73, 4499; Widmann, J. F.;Aardahl, C. L.; Davis, E. J. Aerosol Science and Technology 1997, 27, 636; Richardson, C. B.; Pigg, A. L.; Hightower, R. L. Proc. Roy. Soc. A 1989, 422, 319; Roulleau, M.; Desbois, M. J. Atmos. Sci. 1972, 29, 565; Schweizer, J. D.; Hanson, D. N. J. Colloid and Interface Sci. 1971, 35, 417; Berg, T. G. O.; Trainor, R. J.; Vaughan, U. J. Atmos. Sci. 1970, 27, 1173; Ataman, S.; Hanson, D. N. Ind. Eng. Chem. Fundam. 1969, 8, 833; andDoyle, A.; Moffett, D. R.; Vonnegut, B. J. Colloid Sci. 1964, 19, 136.
High speed photography of charged droplets in an electrospray plume by Gomez and Tang support the prevailing theories of Rayleigh discharge. FIG. 3 shows a charged droplet undergoing jetting in an event attributed to Rayleigh discharge. In this event, it is clear that the parent is elongating and emitting a series of fine progeny droplets. Although they did not measure the charge lost during Rayleigh discharge, they noted the occurrence at ˜60 to 80% the Rayleigh limit, and the photograph supports other experimental findings of little mass loss. (See, e.g., Gomez, A.; Tang, K. Phys. Fluids 1994, 6, 404, the disclosure of which is incorporated herein by reference.)
More recently, Duft and co-workers explored Rayleigh discharge through accurate measurements of the quadrupolar oscillations in a droplet suspended in an alternating current electric field. Through a calculation of the Coloumb energy and surface energy of a droplet, they determined that ethylene glycol undergoes discharge at 100% qR without reliance on the bulk surface tension parameter σ to determine qR, allowing multiple characterizations and the development of a time profile of droplet size and charge. (See, e.g., Duft, D.; Lebius, H.; Huber, B. A.; Guet, C.; Leisner, T. Phys. Rev. Lett. 2002, 89, art. no. 084503, the disclosure of which is incorporated herein by reference.) That droplets are repeatedly characterized in a small measurement volume before and after Rayleigh discharge suggests a “soft” event in which little, if any, momentum is imparted to the parent droplet. This work also suggests a solvent dependence on the charge loss and the percent Rayleigh limit at discharge. (See, e.g., Smith, J. N.; Flagan, R. C.; Beauchamp, J. L. J. Phys. Chem. A 2002, 106, 9957, the disclosure of which is incorporated herein by reference.)
The Rayleigh discharge phenomenon is the result of excess electrical pressure within a droplet. In this case, the electrical pressure of high charge drives an instability leading to jetting of fine jets of charged progeny droplets. However, electrical pressure leading to droplet instability may also be due to an applied strong electric field. Concurrent with investigations into the electrospray phenomenon, investigators considered the behavior of electric fields on neutral water droplets. Early researchers saw meteorological implications and focused on how fields within clouds would affect rain and aerosol drops. (See, e.g., Wilson, C. T. R. Phil. Trans. A 1921, 221, 104; Wilson, C. T. R.; Taylor, G. I. Proc. Cambridge Philos. Soc. 1925, 22, 728; Nolan, J. J. Proc. R. Ir. Acad. Sect. A 1926, 37, 28; Macky, W. A. Proc. Roy. Soc. A 1931, 133, 565; O'Konski, C. T.; Thacher, H. C. J. Phys. Chem. 1953, 57, 955; and Taylor, G. Proc. R. Soc. London, Ser. A 1964, 280, 383, the disclosures of which are incorporated herein by reference.) In a classic 1931 experiment, Macky dropped ˜1 to 5 mm diameter water droplets through a strong electric field with the apparatus shown in FIG. 4a. (See, e.g., Macky, W. A. Proc. Roy. Soc. A 1931, 133, 565, the disclosure of which is incorporated herein by reference.) Here a voltage difference between plates (i) and (ii) defines a high electric field. Water droplets are produced from reservoir (iii) through a stopcock (v) and field-free region (iv). Macky observed that strong electric fields caused droplets to elongate into spheroids prolate to the electric field. At a critical field strength Ec0, droplets developed instabilities resulting in the formation of two symmetrical fine filaments from opposing sides of the droplet. FIG. 4b shows this elongation and instability in photographs of 5 mm diameter water droplets exposed to a 8250 V cm−1 electric field. We call this droplet instability and jetting phenomenon field-induced droplet ionization (FIDI). FIG. 5 shows a single 170 μm diameter methanol droplet undergoing FIDI elongation and jetting. (See, e.g., Grimm, R. L.; Beauchamp, J. L. J. Phys. Chem. B 2003, 107, 14161, the disclosure of which are incorporated herein by reference.)
In general terms, excess electrical pressure leading to FIDI develops in droplets of radius r and surface tension σ when the applied field exceeds a critical value, Ec0 known as the Taylor limit, shown in Equation (2), named for G. I. Taylor who pioneered the corresponding theory. (See, e.g., Taylor, G. Proc. R. Soc. London, Ser. A 1966, 291, 159; and Basaran, O. A.; Scriven, L. E. Phys. Fluids A 1989, 1, 799, the disclosures of which are incorporated herein by reference.)
                              E          c          0                =                              1.625                                          (                                  8                  ⁢                                                                          ⁢                  π                                )                                            1                /                2                                              ⁢                                    (                                                2                  ⁢                                                                          ⁢                  σ                                                                      ɛ                    0                                    ⁢                  r                                            )                                      1              /              2                                                          (        2        )            In Equation (2), the fitting constant c has been determined both empirically and theoretically, and the accepted value is 1.625 for liquid droplets in air. (See, e.g., Macky, W. A. Proc. Roy. Soc. A 1931, 133, 565; O'Konski, C. T.; Thacher, H. C. J. Phys. Chem. 1953, 57, 955; Taylor, G. Proc. R. Soc. London, Ser. A 1964, 280, 383; and Inculet, I. I.; Kromann, R. IEEE Trans. Ind. Appl. 1989, 25, 9454, S51, the disclosures of which are incorporated herein by reference.) Assuming droplets always distort into spheroidal shapes, Taylor additionally derived Equations (3) and (4), the general relationship between an applied electric field E<Ec0 and the resulting aspect ratio γ=a/b of the major to minor axis of the spheroid.
                    E        =                              I            2                    ⁢                                                    γ                                                      -                    4                                    /                  3                                            ⁡                              (                                  2                  -                                      γ                                          -                      3                                                        -                                      γ                                          -                      1                                                                      )                                                    1              /              2                                ⁢                                    (                                                2                  ⁢                                                                          ⁢                  σ                                                                      ɛ                    0                                    ⁢                  r                                            )                                      1              /              2                                                          (        3        )                                          I          2                =                                            1                              2                ⁢                                                      (                                          1                      -                                              γ                                                  -                          2                                                                                      )                                                        3                    /                    2                                                                        ⁢            ln            ⁢                          ⌊                                                1                  +                                                            (                                              1                        -                                                  γ                                                      -                            2                                                                                              )                                                              1                      /                      2                                                                                        1                  -                                                            (                                              1                        -                                                  γ                                                      -                            2                                                                                              )                                                              1                      /                      2                                                                                  ⌋                                -                      1                          1              -                              γ                                  -                  2                                                                                        (        4        )            
In Equation (3), the coefficient I2 is a higher-order function of γ represented by Equation (4). Although a simple relation does not exist for γ(E) in the spheroidal approximation, Equation (5) approximates the relationship between γ and E in Equation (3) to within 1% for fields less than 55% of the Taylor limit. (See, Saville, D. A. Annu. Rev. Fluid Mech. 1997, 29, 27, the disclosure of which is incorporated herein by reference.)
                              γ          ⁢                                          ⁢                      (            E            )                          =                              (                          1              +                                                9                  ⁢                  r                  ⁢                                                                          ⁢                                      ɛ                    0                                    ⁢                                      E                    2                                                                    16                  ⁢                                                                          ⁢                  σ                                                      )                    ⁢                                    (                              1                -                                                      9                    ⁢                    r                    ⁢                                                                                  ⁢                                          ɛ                      0                                        ⁢                                          E                      2                                                                            16                    ⁢                                                                                  ⁢                    σ                                                              )                                      -              1                                                          (        5        )            Equation (3) predicts γ increases with increasing E until γ=1.85 where the droplet becomes unstable corresponding to E=Ec0. This relationship is supported by experimental and theoretical evidence for neutral droplets and soap films in air. (See, e.g., Wilson, C. T. R.; Taylor, G. I. Proc. Cambridge Philos. Soc. 1925, 22, 728; and Basaran, O. A.; Scriven, L. E. Phys. Fluids A 1989, 1, 799, the disclosures of which are incorporated herein by reference.)
Using these theoretical predictions it is possible to determine the electrical field necessary to produce charged ions from droplets. For example, FIGS. 6 and 7 explore Taylor's spheroidal approximation as applied to droplets relevant to the current discussion. FIG. 6 shows Equation (3) plotted for 225 μm, 500 μm, and 2.25 mm diameter droplets. Each curve demonstrates that the equilibrium aspect ratio increases with applied field for 0<E<Ec0 and curves inward at higher aspect ratio values. This turning point agrees well with the Taylor limit presented by Equation (2). At the Taylor limit, the equilibrium aspect ratio is approximately 1.85. FIG. 6 demonstrates that for a given applied electric field, larger droplets will be more elliptical than smaller droplets and corroborates Equation (2), which suggests that larger droplets require lower field strengths to become unstable and exhibit jetting.
FIG. 7 plots the equilibrium aspect ratio for 225 μm diameter droplets of methanol and water. For a specific applied field and droplet size, droplets with a lower surface tension have a greater equilibrium aspect ratio. This plot also shows that 225 μm water droplets do not undergo jetting under standard atmospheric conditions because the necessary field is greater than the 3×106 V m−1 dielectric breakdown limit of air. For this droplet, air will break down and arc before the Taylor limit of field is reached. Indeed, inserting Ec0=3×106 V m−1 and σ=0.072 N m−1 into Equation (2), the smallest water droplet that demonstrates jetting and FIDI would be ˜380 μm in diameter. Practical considerations that lower the breakdown limit of air such as humidity and burrs on the electrodes could be used to raise the practical water droplet diameter minimum to about 500 μm.
Additionally, between the Rayleigh limit of charge and the Taylor limit of field exists the general case where excess electrical pressure within a droplet results from both net charge and the externally applied electric field. For a droplet of charge q, this shape becomes unstable at a critical electric field, Ecq. The critical field is a function of net charge as increasing charge reduces the field necessary to create an instability, or 0≦Ecq≦Ec0 for 0≦q≦qR.
Although chemistry on and within liquid droplets is ubiquitous in nature and anthropomorphic processes, investigators have only begun to understand and harness such reactions. Liu and Dasgupta review applications of liquid droplets to problems including window-less spectroscopy, solvent extraction, and trace gas detection. (See, Liu, H. H.; Dasgupta, P. K. Microchemical Journal 1997, 57, 127-136, the disclosure of which is incorporated herein by reference.) Because of the desire to understand the behavior of aerosols in the atmosphere, much of the recent work is devoted to understanding heterogeneous reactions between droplets and reactive gas-phase species. (See, Finlayson-Pitts, B. J.; Pitts Jr., J. N. Chemistry of the Upper and Lower Atmosphere: Theory, Experiments, and Applications; Academic Press: San Diego, 2000, the disclosure of which is incorporated herein by reference.) Popular techniques for studying atmospherically relevant heterogeneous chemistry include falling-drop and aerosol time-of-flight (ATOF) mass spectrometry (MS) experiments. For example, real-time Raman spectroscopy has enabled researchers to examine heterogeneous reactions on single droplets suspended in an electrodynamic balance. (See, e.g., Davis, E. J.; Aardahl, C. L.; Widmann, J. F. J. Dispersion Sci. Technol. 1998, 19, 293-309; Buehler, M. F.; Davis, E. J. Colloids Surf., A 1993, 79, 137-149; Musick, J.; Popp, J. Phys. Chem. Chem. Phys. 1999, 1, 5497-5502; and Musick, J.; Popp, J.; Trunk, M.; Kiefer, W. Appl. Spectrosc. 1998, 52, 692-701, the disclosures of which are incorporated herein by reference.) The Agnes group established the viability of offline mass spectrometric analysis of individual soft-landed droplets that had previously been suspended in an electrodynamic balance (EDB). They further demonstrated that the suspended droplet may undergo heterogeneous reactions that may be subsequently characterized through offline mass spectrometry. (See, Bogan, M. J.; Agnes, G. R. Anal. Chem. 2002, 74, 489-496; and Feng, X.; Bogan, M. J.; Chuah, E.; Agnes, G. R. J. Aerosol Sci. 2001, 32, 1147-1159, the disclosures of which are incorporated herein by reference.) More recently, using a constant stream of droplets generated by a vibrating orifice aerosol generator, it has been demonstrated that the charged progeny droplets emitted from parent droplets in a FIDI event are a viable source of gas-phase ions for mass analysis. (See, e.g., Grimm, R. L.; Beauchamp, J. L. J. Phys. Chem. B 2003, 107, 14161-14163, the disclosure of which is incorporated herein by reference.)
A great deal of work is still needed to develop a complete picture of both electrospray and field-induced droplet ionization mass spectrometry. For example, as a result of the complexity of examining a falling droplet or a spray of droplets few studies have probed the chemical aspects and implications of charged droplet phenomena. Specifically, because of their inherently transient nature, these droplet streams are difficult to manipulate and utilize as chemical probes. In addition, although Taylor's analysis predicts the field necessary for droplet instability and jetting through Equation (2), the analysis does not predict the timescales or the dynamics of the process. Accordingly more efficient ion sources and applications are needed for understanding and refining the use of droplets as a chemical probe, as well as new applications to mass spectrometry. These applications motivate the present disclosure.